Prediction of the sound pressure at the ear drum for open fittings

The fitting of hearing aids requires knowledge of the sound pressure generated at the ear drum. Traditionally, the sound pressure at the ear drum is estimated by the use of a model of an average ear canal (e.g. a coupler), but obviously, such a model cannot account for inter-individual differences. A better practice, but difficult, is the measurement at the ear drum with a probe-microphone. Alternatively, the sound pressure at the ear drum can be predicted by measurements away from the ear drum. Two methods, one of them based on the phase of the reflectance measured at the ear mold, and the other one based on minima of the sound pressure measured in the ear canal a few millimeters away from the ear mold, were investigated for the use with vented hearing aids. The methods to predict the sound pressure at the ear drum for vented hearing aids will be presented. A preliminary validation by probe tube measurements in 20 ears shows that the accuracy of the predictions is close to what could be obtained with closed fittings, with a few excaptions that will be discussed.